In optical, solar energy and similar devices, it is desirable to incorporate dyes into inorganic glasses. Organic dyes are far superior to inorganic dyes in many respects. For example, there is usually a greater intensity of light absorption, and an ability to more closely govern the area of the spectrum in which light absorption takes place. Furthermore, there are many more organic dyes to choose from. However there is a major difficulty that must be overcome for organic dyes to be incorporated into inorganic glasses; viz., the insolubility of organic dyes in the inorganic phase. In the art, this problem has not been satisfactorily overcome, especially for glasses to be employed in special high technology fields, such as those mentioned above. Hence even now, as at the advent of the glassmaking art, colored glasses are commonly made by incorporating colored inorganic oxides into silicate glass.
Recently, there have been attempts to solve the solubility problem by substituting thin plastic sheets for inorganic glasses, and incorporating organic dyes into the plastic. Results have been less than satisfactory, as discussed below.
When dyes are exposed to the light energy they are designed to absorb, they enter into an excited state. If the increase in energy is dissipated by electron transfer, the dyes can react with the plastic matrix in photochemically promoted reactions. This can cause the film to become opaque, or lose a significant physical property such as strength. Furthermore, the interaction can cause a change in the chemical composition of the dye with attendant loss of desired function.
Avnir et al, Journal of Non-Crystalline Solids 74 (1985) 395-406, suggested means to overcome the problems discussed above. They trapped certain organic dyes in silica and silica-titania thin films by the sol-gel technique. The sol-gel technique is a low temperature method for forming inorganic glasses; it involves hydrolysis and condensation reactions of a material such as tetramethoxysilane. In the process, the methoxy groups are hydrolyzed with water in the presence of an acid catalyst. The silanol groups, ##STR1## obtained by hydrolysis, condense to form ##STR2## linkages. Through these linkages, three-dimensional inorganic networks result. In the Avnir system, such networks trap the organic dye. In other words, entrapment is used to overcome the inherent immiscibility of the organic material in the inorganic network.
Although the Avnir proposal would appear to overcome the age-old insolubility problem, it has not been widely adopted. The lack of wide use indicates that there are significant problems in the Avnir system. Specifically, it appears that the Avnir method does not prevent the microaggregation of dye molecules in the inorganic matrix. Aggregates can have light absorption maxima at different wavelengths than the monomeric dye; thus a different color results along with potentially lower stability. These differences in behavior can cause serious detrimental consequences in the photographic, optical and related arts.
Microspheres of dye aggregates in prior art compositions need not be present at the time that the systems are produced; instead they may form over time, as the prior art materials are exposed to use conditions. In prior art materials, the dye molecules are not permanently entrapped within the inorganic matrix. Instead, the molecules have some freedom to migrate. When two or more dye molecules come together through migration, they tend to form aggregate structures where optional properties depend on the specific stacking orientation and interplanar separation of the chromophores.
Furthermore, dye molecules within the interior of the microspheres may be shielded from light, and therefore unable to interact with the light. This causes a loss of function. Moreover, if the dye aggregates grow in size so that more and more dye molecules become shielded, a deleterious fading will take place over time. In sophisticated equipment, such fading is highly objectionable.
The present invention markedly departs from the teachings of Avnir et al. More specifically, in one embodiment, the present inventors provide compositions which comprise organic-modified metal oxide heteropolycondensates rather than the wholly inorganic condensates employed by Avnir et al. In another embodiment, the present inventors have provided improved compositions of wholly inorganic matrices. Moreover, in contrast to Avnir, the present inventors use dye-containing polymers rather than monomeric dyes. Furthermore, in applicants' compositions the dye-containing polymers are covalently linked to the heteropolycondensate network.
It is surprising that optically clear films can be made from the compositions of the invention. Until applicants' work, it would have been expected that the inorganic and organic components would form an opaque system, thereby rendering the compositions unsuitable for use in imaging, photographic, and/or solar energy and related arts. Thus it is believed that the present invention is a significant advance in the art.